What is a genome-wide association study?

A genome-wide association study is an approach that involves rapidly scanning markers across the complete sets of DNA, or genomes, of many people to find genetic variations associated with a particular disease. Once new genetic associations are identified, researchers can use the information to develop better strategies to detect, treat and prevent the disease. Such studies are particularly useful in finding genetic variations that contribute to common, complex diseases, such as asthma, cancer, diabetes, heart disease and mental illnesses.

Why are such studies possible now?

With the completion of the Human Genome Project in 2003 and the International HapMap Project in 2005, researchers now have a set of research tools that make it possible to find the genetic contributions to common diseases. The tools include computerized databases that contain the reference human genome sequence, a map of human genetic variation and a set of new technologies that can quickly and accurately analyze whole-genome samples for genetic variations that contribute to the onset of a disease.

How will genome-wide association studies benefit human health?

The impact on medical care from genome-wide association studies could potentially be substantial. Such research is laying the groundwork for the era of personalized medicine, in which the current one size-fits-all approach to medical care will give way to more customized strategies.

In the future, after improvements are made in the cost and efficiency of genome-wide scans and other innovative technologies, health professionals will be able to use such tools to provide patients with individualized information about their risks of developing certain diseases. The information will enable health professionals to tailor prevention programs to each person's unique genetic makeup. In addition, if a patient does become ill, the information can be used to select the treatments most likely to be effective and least likely to cause adverse reactions in that particular patient.

What have genome-wide association studies found?

Researchers already have reported considerable success using this new strategy. For example, in 2005, three independent studies found that a common form of blindness is associated with variation in the gene for complement factor H, which produces a protein involved in regulating inflammation. Few previously thought that inflammation might contribute so significantly to this type of blindness, which is called age-related macular degeneration.

Similar successes have been reported using genome-wide association studies to identify genetic variations that contribute to risk of type 2 diabetes, Parkinson's disease, heart disorders, obesity, Crohn's disease and prostate cancer, as well as genetic variations that influence response to anti-depressant medications.

How are genome-wide association studies conducted?

To carry out a genome-wide association study, researchers use two groups of participants: people with the disease being studied and similar people without the disease. Researchers obtain DNA from each participant, usually by drawing a blood sample or by rubbing a cotton swab along the inside of the mouth to harvest cells.

Each person's complete set of DNA, or genome, is then purified from the blood or cells, placed on tiny chips and scanned on automated laboratory machines. The machines quickly survey each participant's genome for strategically selected markers of genetic variation, which are called single nucleotide polymorphisms, or SNPs.

If certain genetic variations are found to be significantly more frequent in people with the disease compared to people without disease, the variations are said to be "associated" with the disease. The associated genetic variations can serve as powerful pointers to the region of the human genome where the disease-causing problem resides.

However, the associated variants themselves may not directly cause the disease. They may just be "tagging along" with the actual causal variants. For this reason, researchers often need to take additional steps, such as sequencing DNA base pairs in that particular region of the genome, to identify the exact genetic change involved in the disease.

How can researchers access data from genome-wide association studies?

The National Center for Biotechnology Information (NCBI), a part of NIH's National Library of Medicine, is developing databases for use by the research community. An archive of data from genome-wide association studies on a variety of diseases and conditions already can be accessed through an NCBI Web site, called the Database of Genotype and Phenotype (dbGaP) located at: http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=gap.

What is NIH doing to support genome-wide association studies?

NIH, the Foundation for the National Institutes of Health, Pfizer Global Research & Development and others have formed a public-private partnership, the Genetic Association Information Network (GAIN), to fund genome-wide association studies. After peer-review of applications, GAIN announced its first round of studies in October 2006. The initial studies include bipolar disorder, major depression, kidney disease in type 1 diabetes, attention deficit hyperactivity disorder, schizophrenia and psoriasis. More information about GAIN can be found at: http://www.fnih.org/work/past-programs/genetic-association-information-network-gain.

In addition, individual NIH institutes have started genome-wide association studies. For example, the National Heart Lung and Blood Institute (NLBI) has launched the Framingham Genetic Research Study in collaboration with the Boston University School of Medicine. In that study, 9,000 participants in the long-running Framingham Heart Study will undergo genome-wide association studies to identify the genes underlying cardiovascular and other chronic diseases, such as osteoporosis and diabetes. More information on that study can be found at: http://www.nhlbi.nih.gov/news/press-releases/2006/nhlbi-to-launch-framingham-genetic-research-study.html.

Other NHLBI efforts in this area include genome-wide association studies involving the Women's Health Study, the Women's Health Initiative and the Candidate Gene Association Resource, which pools DNA samples collected from multiple NHLBI cohort studies. NHLBI, along with the National Institute of General Medical Sciences, also are major contributors to the PharmacoGenetics Research Network. Along with many other tools and technologies, this network is using genome-wide association studies to explore the effects of genes on individuals' varying responses to medications.

Some NIH institutes already have completed genome-wide association studies and deposited their data in the NCBI dbGaP database. These studies include research by the National Eye Institute on age-related eye diseases and the National Institute of Neurological Disorders and Stroke on Parkinson's disease.